1. Field of the Invention
The present invention relates generally to oscillator circuits and oscillation control methods, and more particularly to an oscillator circuit that oscillates by charging and discharging a capacitor and an oscillation control method for controlling oscillation caused by charging and discharging a capacitor.
2. Description of the Related Art
In a power supply circuit such as a DC-DC converter, voltage is controlled by PWM (pulse width modulation) control. In this case, a triangle wave is generated by a triangle wave generator circuit, and a detected voltage and the triangle wave are compared so that the output is inverted based on the magnitude relationship between the detected voltage and the triangle wave, thereby generating a pulse for PWM.
FIG. 1 is a circuit diagram showing a conventional triangle wave generator circuit 10.
The triangle wave generator circuit 10 includes a comparator 11 (COMP1), a comparator 12 (COMP2), a latch circuit 13, reference voltage sources 14 and 15, a charging and discharging circuit 16, and a capacitor 17.
The inverting input terminal of the comparator 11 is connected to the reference voltage source 14 so that a reference voltage VREF1 is applied from the reference voltage source 14 to the inverting input terminal of the comparator 11. The non-inverting input terminal of the comparator 11 is connected to an output terminal Tout so that an output triangle wave is fed from the output terminal Tout to the non-inverting input terminal of the comparator 11. The output level of the comparator 11 is LOW when the output triangle wave is lower than the reference voltage VREF1, and is HIGH when the output triangle wave is higher than the reference voltage VREF1. The output of the comparator 11 is fed to the reset input (R) of the latch circuit 13.
The non-inverting input terminal of the comparator 12 is connected to the reference voltage source 15 so that a reference voltage VREF2 is applied from the reference voltage source 15 to the non-inverting input terminal of the comparator 12. The inverting input terminal of the comparator 12 is connected to the output terminal Tout so that the output triangle wave is fed from the output terminal Tout to the inverting input terminal of the comparator 12. The output level of the comparator 12 is HIGH when the output triangle wave is lower than the reference voltage VREF2, and is LOW when the output triangle wave is higher than the reference voltage VREF2. The output of the comparator 12 is fed to the set input (S) of the latch circuit 13.
The latch circuit 13 is formed of an R-S latch. The output level of the latch circuit 13 is LOW when the output of the comparator 11 switches from HIGH to LOW, and is HIGH when the output of the comparator 12 switches from HIGH to LOW. The output of the latch circuit 13 is fed to the charging and discharging circuit 16.
The charging and discharging circuit 16 includes transistors M1 through M6 and a constant current source 21. The transistors M1 and M2 form a current mirror circuit. The transistor M2 has current supply capacity four times that of the transistor M1. The transistor M2 supplies a current 4I, four times a current I flowing through the constant current source 21, to the source of the transistor M3. The drain of the transistor M3 is connected to one end of the capacitor 17. When the output level of the latch circuit 13 is LOW, the transistor M3 turns ON so as to supply the one end of the capacitor 17 with the current 4I supplied from the transistor M2, thereby charging the capacitor 17. The other end of the capacitor 17 is connected to ground.
Likewise, the transistors M1 and M4 also form a current mirror circuit. The transistor M4 has current supply capacity twice that of the transistor M1. The transistor M4 supplies a current 2I, twice the current I flowing through the constant current source 21, to a current mirror circuit formed by the transistors M5 and M6.
The transistors M5 and M6 draw a current corresponding to the current 2I flowing through the transistor M4 from the one end of the capacitor 17, thereby discharging the capacitor 17 with the current 2I. The capacitor 17 is charged with the current 2I when the transistor M3 is ON, and is discharged with the current 2I when the transistor M3 is OFF.
FIG. 2 is an operational waveform chart of the conventional triangle wave generator circuit 10. In FIG. 2, (a) indicates the output triangle wave of the output terminal Tout, (b) indicates the output of the comparator 11, (c) indicates the output of the comparator 12, and (d) indicates the output of the latch circuit 13.
At Time t0, the triangle wave becomes higher than the reference voltage VREF1, and the output level of the comparator 11 becomes HIGH.
When the triangle wave becomes higher than the reference voltage VREF2 at Time t1, the output level of the comparator 12 switches from HIGH to LOW. As a result, the latch circuit 13 latches the output of the comparator 11 at the time, so that the output level of the latch circuit 13 switches to HIGH. When the output level of the latch circuit 13 switches to HIGH, the transistor M3 turns OFF, so that the capacitor 17 is discharged with the current 2I. As a result, the triangle wave falls.
When the capacitor 17 is discharged so that the triangle wave becomes lower than the reference voltage VREF1 at Time t2, the output of the comparator 11 switches from HIGH to LOW. When the output of the comparator 11 switches from HIGH to LOW, the latch circuit 13 is reset, so that the output level of the latch circuit 13 becomes LOW. As a result, the transistor M3 turns ON, so that the capacitor 17 is charged with the current 2I. In consequence, the triangle wave rises.
The triangle wave is formed by repeating the above-described operation.
In recent years, there has been a demand that DC-DC converters have the capability to generate different voltages. In order to realize such capability with a simple configuration, it is necessary to form multiple triangle waves of different frequencies in synchronization with each other in the DC-DC converter. However, according to the conventional triangle wave generator circuit, it is difficult to synchronize triangle waves with each other because of variations in capacitors and charging currents charging the capacitors.